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Sky at a Glance | December 21st, 2012

As the Moon nears full, it lands between Jupiter and Aldebaran on Christmas evening for the longitudes of the Americas. European observers: move each Moon symbol a quarter of the way toward the one for the previous date. The Moon is shown three times actual size.

By about 9 p.m. at this time of year (depending on how far east or west you live in your time zone), the dim Little Dipper hangs straight down from Polaris, as if from a nail on the cold north wall of the sky. If you have a light-polluted sky, all you may see are Polaris and the two stars forming the far end of the Little Dipper's bowl: Kochab and Pherkad, the "Guardians of the Pole."

Winter begins in Earth's Northern Hemisphere at the solstice, 6:12 a.m. EST, when the Sun begins its six-month return northward. This is the shortest day of the year and the longest night. (But the Earth didn't flip over? No continents flying loose? Drat.)

Saturday, December 22

Every year during Christmas season, Sirius rises in the east-southeast, far below Orion, around 7 or 8 p.m. When Sirius is still low, binoculars often show it twinkling in vivid colors. All stars do this when low. But Sirius is the brightest, making the effect more pronounced.

Sunday, December 23

With the coming of winter, the Great Square of Pegasus is once again balancing on one of its corners as it descends the western evening sky.

It's a busy evening at Jupiter. Ganymede, Jupiter's largest satellite, emerges from behind Jupiter's eastern limb at 7:26 p.m. EST — then just 6 minutes later it disappears into eclipse by the planet's shadow. Ganymede emerges from Jupiter's shadow farther out from the planet at 9:42 p.m. EST. Twenty-two minutes after that, Io starts crossing the planet's face, followed by Io's tiny black shadow at 10:36 p.m. EST.

And at 10:45 p.m. EST, Jupiter's Great Red Spot should cross the planet's central meridian.

For all of Jupiter's satellite events and all the Great Red Spot's transit times, get our new JupiterMoons app.

Monday, December 24

The Moon, nearly full, shines upper right of Jupiter and Aldebaran early this evening, as shown above. Look too for the Pleiades closer left of the Moon. By around 9 p.m. Jupiter and the Moon are level with each other, when they're both very high overhead.

Tuesday, December 25

Christmas conjunction of the Moon and Jupiter, with Aldebaran right nearby! Jupiter and the Moon are only a degree or two apart, depending on your location (in the Americas) and the time of evening. Watch them change separation and orientation as the night progresses. Binoculars show stars of the big, loose Hyades cluster in their background.

Wednesday, December 26

Now the Moon is lower left of Jupiter in the evening, as shown above. The Moon is in the area of Beta and especially Zeta Tauri, the horn tips of Taurus.

Thursday, December 27

Full Moon tonight (exact at 5:21 a.m. Friday morning Eastern Standard Time). The Moon is in the top of Orion's dim club, just under the feet of the Castor figure in Gemini.

Friday, December 28

Look left of the Moon this evening for Castor and Pollux, one above the other. Castor is the one on top. Farther right of the Moon is Orion.

The two brightest asteroids are looping near Jupiter. Click the image for our detailed finder chart, big and printable for use outdoors.

Tony Flanders

Saturday, December 29

Once it rises after dinnertime, the Moon forms a gently curving arc with Pollux and Castor to its upper left, and Procyon to its right or lower right.

The two leading asteroids, Ceres and Vesta, are still in good binocular range at magnitudes 7.1 and 6.9, respectively. Vesta is near Jupiter and Aldebaran, and Ceres is right between the horns of Taurus not far away. Locate them using our finder chart in the December Sky & Telescope, page 50, or online. The light of the waning Moon will be less and less of an issue in the coming nights.

Want to become a better amateur astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.

For an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential guide to astronomy. Or download our free Getting Started in Astronomy booklet (which only has bimonthly maps).

The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6  which may sound like a lot, but that's less than one star in an entire telescopic field of view, on average. By comparison, Sky Atlas 2000.0 plots 81,312 stars to magnitude 8.5, typically one or two stars per telescopic field. Both atlases include many hundreds of deep-sky targets  galaxies, star clusters, and nebulae  to hunt among the stars.

Can a computerized telescope replace charts? Not for beginners, I don't think, and certainly not on mounts and tripods that are less than top-quality mechanically (able to point with better than 0.2° repeatability). As Terence Dickinson and Alan Dyer say in their invaluable Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."

This Week's Planet Roundup

All week at dawn, Mercury shines down to the lower left of bright Venus. By December 21st, try also for Antares, as shown here. Binoculars help.

This week Venus gradually sinks lower in the dawn, Mercury below it sinks a little faster, and sparkly Antares climbs higher  beginning an apparition that will bring it into the evening sky next summer.

Sky & Telescope

Mercury (magnitude –0.5) can be spotted during dawn, well to the lower left of brilliant Venus low in the southeast. They're 8° or 9° apart.

Venus (magnitude –3.9) is still the bright "Morning Star," but it's moving lower in the dawn every week. Look southeast.

Mars (magnitude +1.2) still remains low in the southwest in evening twilight. In a telescope it's just a tiny blob 4.3 arcseconds in diameter — hardly larger than Uranus!

Jupiter (magnitude –2.8, in Taurus) is up and glaring in the east as twilight fades. It climbs to dominate the high eastern and southeastern sky in the evening, with orange Aldebaran 5° below it and the Pleiades twice as far to its upper right. Jupiter is highest in the south around 10 p.m. local time. In a telescope it's still a big 47 arcseconds wide.

Saturn (magnitude +0.7, in Libra) rises in the east-southeast around 3 a.m. local time. By the beginning of dawn it's fairly high in the southeast, far upper right of Venus. That's the best time to get your telescope on it. Saturn's rings are now tilted 19° to our line of sight, the widest open they've been in seven years.

All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America. Eastern Standard Time (EST) equals Universal Time (also known as UT, UTC, or GMT) minus 5 hours.

Like This Week's Sky at a Glance? Watch our SkyWeek TV short. It's also playing on PBS!

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Plan for clear skies with our 2015 observing calendar! Each month features stunning astrophotography and sky scenes that illustrate the positions of the Moon and bright planets. Plus, keep track of important celestial events, such as eclipses, meteor showers, and conjunctions.